The present invention relates to a collectorless direct current motor equipped with a fan or for driving a fan and including a permanent magnet rotor in the field of at least one stator winding and to a method of operating such a motor. In particular, the invention relates to a driver circuit for a collectorless direct current motor including a permanent magnet rotor having at least two poles and at least one stator winding connected to the driver circuit end stage which temporarily operates as a switch and a sensor detecting the position of the rotor, with the control signal fed to the end stage during each commutation phase causing the current in the stator winding to have a ramp-shaped configuration.
Such a driver circuit is disclosed in No. DE-OS 3,107,623 and includes an RC member with the aid of which rectangular signals are reshaped to control the direct current motor in order to reduce the steepness of their edges, thus reducing the winding noise of the motor. However, in the known driver circuit, there exists neither a possibility to change the number of revolutions nor a possibility to regulate the number of revolutions as a function of an external physical value independently of the operating voltage.
It is known to detect the position of the rotor by means of at least one galvanomagnetic element, a Hall generator or the like, and to use the signal generated in this element, which is a function of the rotor position, to control, by way of semiconductor elements, the currents in one or a plurality of stator windings.
The control circuits employed for this purpose are supplemented by members which regulate the number of revolutions as a function of externally detectable physical values with an otherwise constant operating voltage. Regulation of the number of revolutions may be controlled as a function of various parameters or it may involve an adjustment of the number of revolutions of a fan driving motor which provides ventilation that is automatically adapted to demands for a stream of air. In this case, the fan may be part of a device to be cooled which heats up to different temperatures and whose heat is to be dissipated by the fan. In that case, the heat to be dissipated would be the external physical command variable which determines the regulation of the number of revolutions.
Not only for this exemplary case of use but quite generally, users or manufacturers of such direct current drives desire to make available the smallest possible motors for their space-saving advantages. Power losses should be kept low.
To vary the output of motors operated with a constant operating voltage, it is known to pulse width modulate the motor current. In this case, a low pulse frequency in the audible frequency range is selected. Such a frequency does not produce much additional power loss and does not radiate much interference onto adjacent devices, but it does have the drawback of developing a considerable amount of additional noise. Therefore it is also known to select a high pulse frequency in order to reduce noise. Then, stray high frequency fields result which interfere with the devices with which the fans and the corresponding direct current drives are associated. The simplest regulation employs a rough turn-on and turn-off range with the drawback of restless, rumbling motor operation. The demand for small structures gives rise to the additional desire to integrate the components employed in the circuits and to combine them in a chip. Therefore, the power losses in the integrated active and passive components employed must be kept low so that the components can be placed in close, juxtaposition and encapsulated. This demand is counter to the necessity of allowing sufficient current to flow in the electromagnetic peaks then occur in the control circuits and the heat generated by these peaks must not be permitted to destroy the integrated electronic components.
It is an object of the invention to make available a driver circuit for a collectorless direct current motor so as to permit changes in output power and number of revolutions with small amounts of circuitry and at a constant operating voltage; power losses in the components should be low, particularly in the semiconductor paths and here again particularly in the amplifying semiconductor paths, e.g. the end stage transistors. Moreover, additional motor noise and stray high frequency interferences should be kept low.
To solve these problems it is proposed, in general, to permit the active components, i.e. inserted semiconductor paths, to act differently in the same circuit in dependence on given external conditions or external physical command variables. For example, some of the semiconductor elements for controlling the currents for the stator winding or windings may operate as analog amplifier elements and, near the maximum possible number of revolutions, as switches, to thus shorten the turn-on duration, for example, during a commutation phase. As one feature of this solution, ramp-shaped current curves can be generated in the circuit over time, along which the switch positions may be varied. Another possible solution can be realized by means of a delay circuit which delays the moment of turn-on given by the position detector as a function of the external command variable.
As a whole, the invention is based on the idea of avoiding power losses in all components. Thus it becomes possible to integrate the components. The external conditions for the different operating states are detected by at least one component. It is known to employ temperature sensitive components in a ventilation stream to regulate the number of revolutions or power of a collectorless direct current motor used in conjunction with 9 a fan. This, however, is effected by directly setting the power output in the end stage semiconductors. Although this does not produce any additional high frequency fields which could interfere with the devices to be ventilated and such a regulation does not produce any additional noise, a considerable amount of additional power loss develops in the partial load range concomitant with heating of the semiconductor paths. This heating is an impediment for integration of the components. The present invention is based on the realization that, over long xe2x80x9conxe2x80x9d periods, these motors are driven only in the partial load range. According to the invention, the motor current is therefore regulated within this range, for example by way of pulse width modulation with a frequency equal to the commutation frequency of the motor and by operating the actuated end stage semiconductor within one turn-on phase as a switch and subsequently in an analog mode. During this analog operation, the motor current can be reduced, for example, according to a ramp function generated in the driver circuit.
The combination of switch operation and analog operation, preferably in connection with a low rate of revolution at synchronous switching frequencies, permits a significant reduction of the losses in the semiconductor paths and also of electromagnetic and acoustic interferences.
As a further feature of the present invention, the instant at which the motor current is turned on, when the motor is under partial load, is delayed with respect to the turn-on instant which is indicated by a position indicator. This produces a further smoothing of motor operation. Evidently two effects are here at work. The generation of a parasitic axial force which has its maximum in the commutation range is reduced. Secondly, the counter-emf which rises rapidly after the theoretical moment of communutation now considerably reduces the current rise rate so that in spite of pure switch operation, the current increases relatively slowly.